An electric power steering apparatus having a motor for a steering system has been known, which employs a control unit for controlling the power supplied by the motor, relieves the steering torque required of a driver.
In the apparatus, an Electric Control Unit (ECU) determines target current to be supplied to the motor according to steering torque and the speed of a vehicle, then comparing the target current with the actual current flowing into the motor. The apparatus consequently controls the motor based on a deviation resulting from the comparison.
ECU has a microcomputer for controlling an electric power steering apparatus. The microcomputer controls feedback current so that the target and actual current can be in agreement; in other words, the deviation can be zero. In a feedback loop of current including the ECU and microcomputer, PID control is adopted in order to make the deviation converge to zero fast with a higher response
However, the feedback control described above does not necessarily realize a sufficiently high response to the change of motor current. A method disclosed in Japanese Published Patent Applications 2001-287658 and 2002-234457 employs a combination of feedback and feedforward control. The feedback control adapts a motor drive signal to be greater according to a deviation and the feedforward control adapts the motor drive signal to be greater according to target current, respectively. In this way, the method improves a response to steering operation.
Feedforward control of this type has a feature that it generates a feedforward control element based on target current and deliverers the element directly. For example, when target current is small, the feedforward control generates a motor drive signal proportional to the target current and delivers the signal to a motor drive circuit. In this way, the addition of feedforward control improves the response to steering, compared with a method employing feedback control only.
However, the method still poses problems when the gain of feedforward control is increased so as to improve the response of combined feedforward and feedback control. The problems include heat dissipated by a motor, melting of a relay contact and the like due to a sudden increase in current resulting from an overshooting caused by a step steering input. In this connection, the step steering input occurs when a steering wheel is steered to an extreme position to cause a rack to strike an end-of-stop during low speed running of a vehicle or sudden steering is made in the reverse direction of large centrifugal force during high speed running of the vehicle.
On the other hand, adopting a measure to increase stability will decrease the response to steering, thereby degrading the steering feeling. Furthermore, the measure brings another problem that it takes long to damp overshooting and vibration once they occur. As discussed above, the response and stability have not been made compatible yet each other.
Also another problem exists that natural vibration is likely to occur if gain is increased.